THE BIOLOGY OF AMBROSIA TRIFIDA L.

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Abstract
Summary: Ambrosia trifida L. is a fast growing annual colonizer of disturbed ground. When present it drastically reduces kinds, numbers and growth of other annuals. Its seedlings emerge before any other annual in early spring. We studied seed germination in relation to temperature, moisture, depth of sowing, and seedling emergence, survivorship and performance of mature plants in relation to time and depth of sowing and seed size.Seeds germinated under a wide range of temperature (8 to 41°C) with an optimum between 10 to 24 °C. Further germination occurred after restratification suggesting germination polymorphism. The seeds also germinated under a wide range of soil moisture conditions (17 to 55 % dry wt) but highest germination occurred at 20 to 33 % soil moisture. Optimum depth of sowing was 2 cm; very little germination occurred very close to soil surface, and some occurred at the 16 cm depth. In the field seedlings emerged from shallow depths first and then from progressively deeper parts of the soil bank. There was positive correlation between seed size and maximum depth from which seeds of this species and those of five other associated annuals emerge. Time required for seedlings to emerge after germination was directly related to depth. Seedlings that emerged first had the highest probability of survival and those that emerged late suffered progressively higher mortality, irrespective of whether late emergence was caused by late germination or deep germination. Among the survivors that reached maturity, plant weight, height, and number of seeds per plant declined with delayed emergence. However, experimental manipulation showed that delayed emergence per se was not the cause of the mortality but rather competition with individuals which emerged earlier. Furthermore, delayed emergence of cohorts in separate plots did not significantly reduce plant height, weight and number of seeds per plant.Thus in the field and within a patch there may be strong selection for early germination and emergence. However, this selection may be opposed by the unpredictability of the time of disturbance, the spatial patchiness of disturbance, and the longevity and induced dormancy of Ambrosia seeds. Compared to its associated annuals, A. trifida has the largest seeds and seedlings, the earliest germination and emergence and a very high photosynthetic rates. These properties give A. trifida a decisive advantage over the other species.